1. Field of the Invention
The present invention relates to an apparatus for detecting foreign substances such as minute dusts, and more particularly to such apparatus for detecting foreign substances attached on a planar substrate such as photomask or reticle for large-scale integrated circuit.
2. Description of the Prior Art
In the process of producing photomasks or wafers for large-scale integrated circuits, foreign substances may become attached to the reticle or masks and eventually cause defects in the produced masks or wafers. In a pattern exposure apparatus for reduction exposure of a reticle pattern onto a mask or of a mask pattern onto a wafer, there are produced plural chip patterns on a single mask or wafer, and strict inspection has to be carried out for such foreign substances in the production process since such defect appears commonly in all chip patterns of the mask or wafer. For this purpose there is usually considered visual inspection, but such inspecting operation requires many hours, thus leading to the fatigue of the inspecting personnel and to the loss of inspecting efficiency.
Recently there are therefore proposed various apparatus for automatically detecting such foreign substances attached to the mask or reticle for example by laser irradiation. The present inventors already proposed, in the U.S. patent application Ser. No. 343,552 filed Jan. 28, 1982, now Pat. No. 4,468,120, issued Aug. 28, 1984, an improvement on such apparatus to enable exact automatic detection of the foreign substance on the glass, thus allowing to identify whether the foreign substance is present on an opaque area such as chromium or on a translucent area of the glass, and, in the latter case, whether the foreign substance is present on a surface of the inspected article at the incident side of the laser beam or on the other surface. It is however not yet possible to identify the attaching state of the foreign substance, namely the shape thereof or the status thereof on the surface, nor to distinguish a scar on the glass surface from the foreign substance.